The present invention relates to data transmission systems and, more particularly, to the cancellation of cross talk interference at a receiver.
Bi-directional digital data transmission systems are presently being developed for high-speed data communication. One standard for high-speed data communications over twisted-pair phone lines that has developed is known as Asymmetric Digital Subscriber Lines (ADSL). Another standard for high-speed data communications over twisted-pair phone lines that is presently proposed is known as Very-High-Speed Digital Subscriber Lines (VDSL).
The Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, has finalized a discrete multi tone based approach for the transmission of digital data over ADSL. The standard is intended primarily for transmitting video data and fast Internet access over ordinary telephone lines, although it may be used in a variety of other applications as well. The North American Standard is referred to as the ANSI T1.413 ADSL Standard (hereinafter ADSL standard). Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of up to 8 million bits per second over twisted-pair phone lines. The standardized system defines the use of a discrete multi tone (DMT) system that uses 256 “tones” or “sub-channels” that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is defined as transmissions from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user). In other systems, the number of tones used may be widely varied. However when modulation is performed efficiently using an inverse fast Fourier transform (IFFT), typical values for the number of available sub-channels (tones) are integer powers of two, as for example, 128, 256, 512, 1024, 2048 or 4096 sub-channels.
The ADSL standard also defines the use of a reverse signal at a data rate in the range of 16 to 800 Kbit/s. The reverse signal corresponds to transmission in an upstream direction, as for example, from the remote location to the central office. Thus, the term ADSL comes from the fact that the data transmission rate is substantially higher in the downstream direction than in the upstream direction. This is particularly useful in systems that are intended to transmit video programming or video conferencing information to a remote location over telephone lines.
ANSI is producing another standard for subscriber line based transmission systems, which is referred to as the VDSL standard. The VDSL standard is intended to facilitate transmission rates of at least 25.96 Mbit/s and preferably at least 51.92 Mbit/s in the downstream direction. To achieve these rates, the transmission distance over twisted pair phone lines must generally be shorter than the lengths permitted using ADSL. Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a similar system, which is referred to as Fiber To The Curb (FTTC). The transmission medium from the “curb” to the customer premise is standard unshielded twisted-pair (UTP) telephone lines.
A number of multi-carrier modulation schemes have been proposed for use in the VDSL and FTTC standards (hereinafter VDSL/FTTC). One proposed multi-carrier solution utilizes discrete multi-tone signals in a system that is similar in nature to the ADSL standard. Other proposed modulation schemes include carrierless amplitude and phase modulated (CAP) signals and discrete wavelet multi-tone modulation (DWMT). In order to achieve the data rates required by VDSL/FTTC, the transmission bandwidth must be significantly broader that the bandwidth contemplated by the ADSL. By way of example, the discrete multi-tone system adopted for ADSL applications utilizes a transmission bandwidth on the order of 1.1 MHz, while bandwidths on the order of 20 MHz are being contemplated for VDSL/FTTC applications. In one proposed DMT system for VDSL/FTTC applications, the use of 256 “tones” or “sub-channels” that are each 69 kHz wide is contemplated. In another proposed DMT system for VDSL/FTTC applications, the use of 4096 “tones” or “sub-channels” that are each 4.3125 kHz wide is contemplated.
The use of these wide band multi-carrier modulation approaches have some inherent obstacles that must be overcome. One particular problem relates to cross talk interference that is introduced to the twisted pair transmission line and received by the modem. As is well known to those skilled in the art, cross talk interference is unwanted interference (signal noise) that is passed between adjacent network cables or devices. Cross talk generally occurs due to coupling between wire pairs when wire pairs in the same bundle are used for separate signal transmission. In this manner, a data signal from one source may be superimposed on a data signal from a second source. As can be appreciated, the data signals being transmitted over the twisted-pair phone lines can be significantly degraded by the cross talk interference generated on an adjacent twisted-pair phone line. As the speed of the data transmission increases, the problem worsens. For example, in the case of VDSL signals being transmitted over the twisted-pair phone lines, the cross talk interference can cause significant degradation of the VDSL signals.
The undesired cross talk interference can come from a variety of sources. One particular source of cross talk interference is from home phone network devices that use existing twisted-pair phone wiring for networking. By way of example, the Home Phone Network Alliance (HomePNA or HPNA) is one organization dedicated to solutions for in-home, phone line-based networking. The HPNA signal is a single carrier signal that tends to occupy a band that is less than approximately 6 MHz wide. As can be appreciated, the band may overlap some portion of the VDSL band, and as a result, the VDSL and HPNA signals tend to occupy some of the same frequency bands. By way of example, HPNA signals generally operate with a frequency between about 4 to about 10 MHz. Accordingly, with high speed data transmission, the cross talk interference produced by HPNA signals and other sources can significantly degrade the desired VDSL data signals being transmitted over twisted-pair phone lines.
Hence, the problem with using twisted-pair phone lines with high frequency data transmission rates, such as available with ADSL and VDSL, is that the cross talk interference becomes a substantial impediment to a receiver being unable to properly receive the transmitted data signals. Thus, there is a need to provide techniques to eliminate or compensate for cross talk interference.